This invention relates generally to extractive distillation processes and extractive distillation compositions for enhancing the separation of hydrocarbon compounds via extractive distillation.
It is difficult to efficiently and economically separate mixtures of organic compounds having similar chemical characteristics, and nearly the same boiling point. Conventional fractional distillation can be expensive and inefficient in this situation, in that large and expensive columns are required, which have large numbers of plates, and utilize high reflux ratios, with correspondingly high energy consumption rates.
Extractive distillation is a technique for separating certain close boiling mixtures. In extractive distillation, a high boiling solvent is typically introduced into a distillation zone containing a lower boiling feed mixture that is to be separated. The high boiling solvent interacts with the feed mixture to effectively decrease the volatility of some feed mixture components, typically the more polar feed components, so that the less polar feed components can be separated as a vapor stream. The solvent and the more polar feed components typically exit the distillation zone as a heavy fraction. An extractive distillation process has been described in the article entitled xe2x80x9cExtractive Distillation Saves Moneyxe2x80x9d by Ian Sucksmith, Chemical Engineering, Jun. 18, 1982, pages 91-95. Other literature sources relating to continuous distillation techniques and/or extractive distillation techniques include Sections 1.1 and 1.2 of the xe2x80x9cHandbook of Separation Techniques for Chemical Engineersxe2x80x9d, Phillip A Schweitzer Editor McGraw Hill Book Company, 1979, and Seader et. al. in Perry""s Chemical Engineering Handbook, pages 13-53 to 13-57, McGraw-Hill Book company, 1984.
A variety of methods for employing extractive distillation to separate various classes of hydrocarbons, such as aromatic, olefins, or cycloalkanes, from other close-boiling hydrocarbons, such as paraffins, is known in the hydrocarbons industry. In particular, many refinery streams comprise xe2x80x9cBTXxe2x80x9d streams composed of close boiling mixtures of aliphatic hydrocarbons (such as isomers of heptane and octane) and aromatic hydrocarbons (such as benzene, toluene, and xylenes). It is known in the industry to use mixtures of solvents such as various polyethylene glycol ethers and water as solvents for extractive distillation of hydrocarbon and/or BTX streams (i.e. xe2x80x9cUdexxe2x80x9d processes, and variations thereof). U.S. Pat. Nos. 3,714,033 and 4,921,581 disclose the use of polyalkylene glycol solvents toward this end.
Since the institution of xe2x80x9cUdexxe2x80x9d technology, efforts have been made to improve its production efficiency and economic performance in hydrocarbon separations by the use of other types of solvents for extractive distillation. Morpholine derivatives and N-alkylpyrrolidones were disclosed as extractive distillation solvents, respectively, in U.S. Pat. Nos. 4,081,355 and 4,948,470. U.S. Pat. No 4,676,872 discloses the use of adiponitrile in combination with other materials such as ethylene carbonate, nitrobenzene, and certain dialkyl esters, for the separation of xylene isomers. U.S. Pat. No. 4,292,142 discloses the use of phthalic anhydride in combination with other materials such as isophorone, for the separation of ethylbenzene from xylenes.
Other known processes have employed organic sulfones as solvents in extractive distillation processes. U.S. Pat. Nos. 2,033,942 and 2,831,039 described the use of dialkyl sulfones, including dimethyl sulfone, in such separations. U.S. Pat. No. 4,401,517 relates to the use of C4-C8 sulfones as selective extractive distillation solvents. U.S. Pat. No. 3,146, 190 described the use of sulfolane (tetramethylene sulfone) as a selective extraction solvent for the purification of pyrolysis fuels and catalytically reformed gasolines. U.S. Pat. No. 3,466,346 describes further refinements of sulfolane based extractive distillation processes.
Polyalkylene glycols or sulfolane are the most widely used extractive distillation solvents used in modern hydrocarbon processing. Both solvents exhibit reasonably high selectivity for separating aromatic materials from aliphatic compounds, but, Wu et. Al. estimate (Chemical Engineering, page 139, March 1998) that for a typical industrial BTX extraction unit, a 1% increase in aromatics recovery would result in savings of up to $100,000 per year. Such savings derive from both decreased need for capital investment, and decreased unit energy requirements (electricity, steam, etc.) needed to perform the extractive distillation. Polyalkylene glycols also suffer from significant thermal degradation on an annual basis, and therefore necessitate regular makeup of their volume, which incurs significant expense.
Relatively recent approaches to extraction medium modification involve the introduction of cosolvents to glycol extraction media, as a way of enhancing extraction performance. U.S. Pat. No. 5,139,651 discloses the use of polyalkylene glycols and a glycol ether cosolvent to extract aromatics. Such compositions are also described in U.S. Pat. Nos. 4,498,980 and 4,781,820. The use of sulfolane as a cosolvent in polyalkylene glycol solvent systems is described in U.S. Pat. No. 5,310,480. U.S. Pat. No. 5,085,740 discloses a ternary mixture composed of an N-alkyl-2-pyrrolidone, a sulfolane compound, and a glycol. U.S. Pat. No. 5,032,232 relates to mixtures of N-alkyl-2-thiopyrollidone with sulfolane and/or tetraethylene glycol, and their use in extractive distillation processes.
Thus, both technical and financial considerations provide a continuing need for developing novel and improved extractive distillation compositions which exhibit advantages (such as higher selectivity and/or capacity/loading) over known solvents for the extractive distillation of mixtures of close-boiling hydrocarbons.
The instant invention relates to improved extractive distillation compositions, and processes that employ the extractive distillation compositions.
In some aspects, the invention relates to extractive distillation compositions for separating a mixture of hydrocarbon compounds, wherein the extractive distillation composition includes at least one alkylene glycol compound, and at least one compatibility agent, wherein the compatibility agent is selected from materials having solubility parameters such that the extractive distillation composition provides improved separation of the hydrocarbon compounds, as compared to the use of the extractive distillation composition without the compatibility agent.
In other aspects, the invention relates to processes for separating hydrocarbon compounds of similar boiling points by extractive distillation, that employ the extractive distillation compositions of the invention.
Additional advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.